1. Field of the Invention
The present invention relates to a photoelectric smoke sensing and alarming device. More particularly, the invention is concerned with a photoelectric smoke sensing and alarm device capable of discriminating the smoke of a fire from smoke having different flow patterns such as cigarette smoke, dust-suspending air and the like, so as not to be actuated erroneously by the smoke other than that generated by a fire.
2. Description of the Prior Art
There have been proposed and used various types of photoelectric smoke sensing and alarming devices in which the smoke gas generated in the initial stage of a fire is introduced into a chamber accommodating light-emitting and light-receiving elements, so that the smoke is sensed as it interrupts the light to be received by the light-receiving element. In this type of device, the chamber has an opening through which the gas suspending the smoke particles is introduced into a dark chamber. At the same time, the sensing output of the light-receiving element is amplified before it is delivered to the alarm circuit. The sensitivity of the device can be adjusted by changing the gain of the amplifier.
In case of smoke generated by a fire, the smoke is gradually and continuously diffused and spread, and the density or thickness of the smoke increases as the time elapses, while smoke from other sources, such as cigarette smoke and dust-suspending air, exhibits different pattern of flow. However, if the sensing circuit is adjusted to be actuated at a high sensitivity, it cannot discriminate the pattern of the smoke of fire from those of other smoke sources. Consequently, the device often issues an alarm signal erroneously upon sensing smoke other than that of a fire.
The erroneous operation of the device is caused also by a scattering of light in the chamber and by scattered light coming from the outside of the chamber entering the chamber. These are closely related to the ease by which the gas is introduced into the chamber, as well as to the construction for preventing the scattering of the light, and also to other factors such as the sensitivity of the sensing circuit and signal to noise ratio.